Yuma Ag and You: Creation of the Yuma Soils

Bobbi Stevenson McDermott                   April 24, 2016 publication

As we look at the new patchwork quilt of fields in various shades of green, most people do not think about how the farmland in the Yuma and Wellton-Mohawk Valley were created. All of the soils in Yuma County are called alluvial, or water deposited soils. In essence, the productive farmlands we value so much were actually created by erosion!

There are two rivers which contributed to the soils we farm, the Gila River and the Colorado River.

During the eons when man was a minor part of the landscape, these rivers, fed by many other rivers and streams were powerful forces of nature. Watching the news this year, the power of concentrated flows of water have continually been displayed, including this week in Houston, TX. Large flows of water contained to a narrow area have the power to move tremendous amounts of soil. The faster the water moves, the more soil it carries until it reaches a floodplain, or flat area where the water spreads out. As the water slows down, the different soil particles begin to settle out. The largest particles are gravels and sand, which drop out close to the river channel. The silt particles are like flour and stay suspended for a greater period of time and the smallest particles, clay can stay mixed in the water for weeks.

The pattern created by the settling of the soil particles is what creates the different layers of soil we find when a column of soil is removed ground. Along the river banks are sandy beaches. Further away from the river edge, the silts and clays accumulate. Depending on the flood flows and where the water comes from, sands can be deposited on top of the silts and clays, creating layers. The sands, if they are in a depression can also be covered with silts and clays as the water stands and the particles settle out.

Yuma does not have a lot of fields that are all the same kind of soil. Even with early development in Yuma County, the Gila and Colorado Rivers carried tons of soils. As farmers diverted the water to irrigate, the soil particles settled onto the farm fields. The building of dams on the Gila and Colorado significantly changed the amount of soil making it to Yuma County. The smaller amounts of water released below the dams and the fact that much of the soil settled out behind the dams reduced soil particles reaching Yuma to almost nothing. On days when the Colorado River is muddy now, it is usually from rain storms that occur between the dams on the river and the desert washes flooding, bringing soil and debris into the river.

The elevation in the Yuma and Gila Valleys is very near to sea level so the water in the wild river days could not flow west, rather going south into Mexico.

The Gila River flows from New Mexico through Clifton, Morenci, the San Carlos Apache Reservation; Hayden, Florence, south of Phoenix where it is joined by the Salt River, through Buckeye and to Gila Bend and the Painted Rock Dam. It is rare that there are significant flows below Painted Rock Dam, a regulating reservoir. The Colorado River enters the state through Big Water, Utah; runs through the Grand Canyon turning south at the Nevada border, then through Bullhead City, lake Havasu, Parker and finally to Yuma.

It is pretty amazing to think that our growers are farming soils that were once in the Grand Canyon or from the mining areas in the Sitgreaves National Forest in Greenlee county. It has taken great skill and trial and error to develop the irrigation systems and croplands covering close to 180,000 acres of Yuma County. What a wonderful job they have done!